Application Hints
(Continued)
struction, but the advantages are sometimes well worth the
effort of using point-to-point up-in-the-air wiring. See
Figure
7.
DS008767-17
(a) Inverting Amplifier
DS008767-21
(Input pins are lifted out of PC board and soldered directly to components.
All other pins connected to PC board.)
FIGURE 7. Air Wiring
BIAS CURRENT TESTING
The test method of
Figure 8
is appropriate for bench-testing
bias current with reasonable accuracy. To understand its op-
eration, first close switch S2 momentarily. When S2 is
opened, then
DS008767-18
(b) Non-Inverting Amplifier
DS008767-19
(c) Follower
DS008767-22
FIGURE 8. Simple Input Bias Current Test Circuit
DS008767-20
(d) Howland Current Pump
FIGURE 6. Guard Ring Connections
The designer should be aware that when it is inappropriate
to lay out a PC board for the sake of just a few circuits, there
is another technique which is even better than a guard ring
on a PC board: Don’t insert the amplifier’s input pin into the
board at all, but bend it up in the air and use only air as an in-
sulator. Air is an excellent insulator. In this case you may
have to forego some of the advantages of PC board con-
A suitable capacitor for C2 would be a 5 pF or 10 pF silver
mica, NPO ceramic, or air-dielectric. When determining the
magnitude of I
b
−, the leakage of the capacitor and socket
must be taken into account. Switch S2 should be left shorted
most of the time, or else the dielectric absorption of the ca-
pacitor C2 could cause errors.
Similarly, if S1 is shorted momentarily (while leaving S2
shorted)
where C
x
is the stray capacitance at the + input.
9
www.national.com
NSC [ NATIONAL SEMICONDUCTOR ]
